In this paper, we report the numerical calculation and experimental verification of a quartz-crystal-resonator-based methanol concentration sensor for fuel cell applications. This sensor is composed of a quartz crystal resonator (QCR) and an interdigital capacitor (IDC). Our sensor can detect a very small capacitance change with a resolution of sub fF. The fabricated sensor can measure a methanol concentration with a resolution of 0.008 wt%. In the present study, we found out optimized sensor design (QCR and IDC) for the substrate thickness and the range of the methanol concentration. By the use of this study, the capacitive sensor with higher resolution can be realized, and it can be expected to apply to various types of capacitive sensors. © 2014 The Institute of Electrical Engineers of Japan.